With recent significant popularity in portable electronic devices (in the following, simply called “electronic devices”) such as personal computers, mobile phones, and mobile devices, a demand for batteries for a power source of electronic devices has been increasing significantly. The batteries used in electronic devices are required to be used at room temperature, and also required to have a high battery capacity, a high energy density, and excellent charge and discharge cycle characteristics. Lithium ion secondary batteries are known as an example of such a battery.
Lithium ion secondary batteries include a positive electrode containing a positive electrode active material capable of reversibly absorbing and desorbing lithium ions, a negative electrode containing a negative electrode active material capable of absorbing and desorbing lithium ions, and an electrolyte with lithium ion conductivity. Currently, lithium ion secondary batteries have high-level battery capacity, energy density, and charge and discharge cycle characteristics, and are widely used for a power source of electronic devices. However, in order to achieve further multi-purpose electronic devices, a further high capacity lithium ion secondary battery is demanded.
To achieve a high capacity lithium ion secondary battery, for example, there have been proposed using a silicon compound or a tin compound as the negative electrode active material. The silicon compound includes silicon, silicon oxides, and silicon-containing alloys. The tin compound includes tin, tin oxides, and tin-containing alloys. Since the silicon compound and the tin compound have a very high capacity, by using these compounds, high capacity batteries can be manufactured.
Silicon compounds and tin compounds characteristically expand and contract due to changes in the crystal structure when absorbing and desorbing lithium. Therefore, when a negative electrode includes a negative electrode active material layer containing a silicon compound or a tin compound provided on the negative electrode current collector surface, the negative electrode active material layer expands and contracts during charge and discharge. Along with the expansion and contraction, stress is generated at the interface between the negative electrode current collector and the negative electrode active material layer, which declines the adhesion between the negative electrode current collector and the negative electrode active material layer, causing the negative electrode active material layer to partially detach from the negative electrode current collector. Such a partial detachment spreads to other portions in due course. As the detached portion of the negative electrode active material layer from the negative electrode current collector increases, current collecting performance declines, thereby shortening charge and discharge cycle life.
To solve such problems, Japanese Patent Publication No. 3733065 has proposed a negative electrode for lithium batteries, including a rough-surfaced negative electrode current collector, and an amorphous silicon thin film (negative electrode active material layer). The amorphous silicon thin film is formed at the rough surface of the negative electrode current collector.
The greatest characteristic of this negative electrode is that the amorphous silicon thin film is used as the negative electrode active material layer. In the amorphous silicon thin film, cuts (voids) which extend in the thickness direction thereof are regularly formed, owing to the expansion and contraction during charge and discharge. The amorphous silicon thin film is separated into a plurality of separately independent columns by the cuts, thereby forming an aggregation of the columns. The above-mentioned Japanese Patent Publication further states that the stress generated with the expansion and contraction of respective columns is eased by the cuts, which prevents the detachment of respective columns.
However, because of a relatively large stress generated at the time of forming the cuts, detachment easily occurs at the end portion of the columns adjacent to the cuts. The detachment at the column end portion unavoidably spreads gradually to other portions, even under a state where the stress of expansion and contraction is eased by the cuts.
Even without the occurrence of the detachment at the column end portion, the stress generated at the time of expansion and contraction with charge and discharge is concentrated at the interface between the columns and the negative electrode current collector at the center portion of the columns, and therefore partial detachment of the columns from the negative electrode current collector, and deformation of the negative electrode current collector cannot be avoided.
Therefore, with the technique of the above-mentioned Japanese Patent Publication, detachment of the negative electrode active material layer cannot be sufficiently and reliably prevented. Also, in the technique of the above-mentioned Japanese Patent Publication, the negative electrode active material is limited to a material in which cuts can be formed by charge and discharge, and therefore the negative electrode active material that can be used is limited. Further, the Japanese Patent Publication does not mention at all about techniques to prevent the spread of the detachment, when the negative electrode active material layer is partially detached from the negative electrode current collector.